Nutrient deficiencies of perennials: Part 12 of 12

Pictures related to the nutrient deficiencies series may accessed by viewing the PDF files of the pages that originally appeared in GMPRO magazine: Page 1. Page 2. Page 3. Page 4.

Nutrient Managment

How a grower should develop a perennial-specific nutrient management program depends on several production aspects. Perennials are grown by both traditional greenhouse growers who have expanded their market beyond bedding crops and by nursery growers who are getting into the lucrative color market.

This presents two decidedly different nutrient-management situations. Many field nursery growers rely on controlled-release fertilizers, and controlled-environment growers use injection systems with water-soluble liquid feed, though the lines are blurring.

Another aspect is the genetic improvement (or lack thereof) of perennials for rapid growth. Many annuals have undergone decades of selection and hybridization, so these plants have become fast, efficient crops when produced under optimum conditions.

Many perennial species are simply that – species. They’ve often evolved to succeed in very low-nutrient situations.

Applying a high fertilizer rate to many perennials treated like a traditional bedding crop often results in: rampant and excessive growth (Achillea and Helianthus) or stunted, slow-growing, salt-stressed plants (Campanula and Filipendula).

How much or how little fertilizer?

Applying water-soluble fertilizer at every watering (fertigation) has become the norm for production of nearly all types of herbaceous plants. The alternative “pulse” method of periodic fertilizer application followed by plain-water irrigation has repeatedly been demonstrated as less efficient in terms of nutrient availability and overall plant growth.

The question with perennials becomes, “how much fertilizer?” We answer with the dreaded, “It depends.” Several studies at Virginia Tech and University of Florida-Milton have defined optimal fertilizer rates for tropical and hardy perennial species.

Controlled-release fertilizer

Length of time in production becomes a management factor, especially with hardy perennials. Many species perform best if liners are potted in summer or fall, overwintered to fulfill vernalization requirements, then marketed in spring. This is a six- to nine-month production cycle.

Growers often rely on controlled-release fertilizer to carry these long-term plants from potting to sale. Studies correlate increased temperature with increasing release rates of nearly all types of controlled-release fertilizers. There’s also an initial release, regardless of product brand. Leachate electrical conductivities from a normal application of nine-month or one-year controlled release fertilizer in the first five weeks can exceed 4.0 milliSiemens per centimeter. Under conditions that simulated outdoor containerized production of perennials (a period of 47 weeks), studies at the University of California compared release rates of 12-month slow-release fertilizers. The research found concentrations of most nutrients in leachates were relatively high, but fluctuated frequently during the first third of the study period, and then gradually decreased and stabilized during the last 27 weeks. Leachate concentrations of nitrate nitrogen and phosphorus from all fertilizer types were usually high, especially from week 5 through week 30. Keep these results in mind, especially if you’re potting late in the season. High fertilizer salt levels combined with little to no overwinter watering can be lethal to sensitive perennials.

Many hardy perennial species are sensitive to high salts. The California study points to the need to carefully monitor electrical conductivity levels and provide enough irrigation at these times to bring the electrical conductivity down to an appropriate range.

Management issues

Because fertilizer accounts for such a small fraction of production costs, there is the tendency to provide plants with all the nutrients they need and more. Pollution of surface and groundwater has been a concern for the entire green industry; conscientious growers make every effort to reduce the amount of runoff.

University and industry research continues to explore the most nutrient-efficient yet economically viable options for water and fertilizer application. University of Florida researchers found traditionally heavy application of nitrogen, phosphorus and potassium was unnecessary in field production of caladium tubers. The same production yield results were obtained when phosphorus wasn’t applied.

Frequent growing media testing to monitor and chart soluble salt levels is essential. Options range from the low-tech but highly reliable PourThru method to the more expensive but easy-to-use direct-stick EC probes. The trick to interpreting and acting on the results is twofold: recommendations are given in media soluble salts measurements, not parts per million of fertilizer applied. This way, regardless of fertilizer type or application method, the grower simply adjusts the concentration and amount of fertilizer applied to maintain a ballpark EC range.

The second essential is charting EC over time and making the necessary water or fertilizer-delivery system adjustments, especially with longer-term production cycles.

- Holly L. Scoggins and Jamie Gibson

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Holly L. Scoggins is associate professor, Virginia Tech, Department of Horticulture, (540) 231-5783; perennials@vt.edu. Jamie Gibson is corporate manage,r quality control/technical services, Fafard Inc., (800) 722-7645, jamie.gibson@fafard.com.